Recently, electric powered vehicles such as a hybrid vehicle and an electric vehicle have attracting attention as environmentally friendly vehicles. Such an electric powered vehicle includes a power storage device formed of a secondary battery or the like, and a motor generator as a motor for driving the vehicle, and is capable of generating driving force, upon receiving electric power from the power storage device. The motor generator generates the driving force at the time of starting or acceleration, and converts kinetic energy of the vehicle to electric energy and recovers it to the power storage device at the time of regenerative braking. In order to control the motor generator in accordance with the state of running of the vehicle as described above, a general electric powered vehicle has a power converting device allowing power conversion between the power storage device and the motor generator through switching control of a power semiconductor switching element in an inverter device or the like.
In order to cope with a failure of the power semiconductor switching element in such an inverter device, Japanese Patent laying-Open No. 2000-333468 (hereinafter referred to as Patent Document 1) discloses an inverter device formed of IGBTs (Insulated Gate Bipolar Transistors) as the power semiconductor switching elements (hereinafter also simply referred to as “switching elements”) provided with an element protective part for individually cutting off each IGBT in case of abnormality such as overheating. Further, the document discloses provision of an inverter device adapted to selectively use a mode in which the element protective part executes individual cut-off (self cut-off) and outputs an abnormality signal to an external control unit and a mode in which the abnormality signal is output to the external control unit without executing individual cut-off, thereby preventing degradation of element protecting function and realizing convenience for use.
If, in an electric powered vehicle, a short-circuit failure occurs in the inverter device controlling driving of a motor generator for driving the vehicle, normal running becomes impossible and, in some cases, the vehicle must be towed to a repair garage. If the motor generator is implemented by a permanent magnet motor having a permanent magnet attached to a rotor, back electromotive force generates as the rotor rotates while the vehicle is towed. This must be addressed.
In this regard, Japanese Patent Laying-Open No. 10-257604 (hereinafter referred to as Patent Document 2) discloses a controlling device for an electric vehicle that prevents damage caused by the back electromotive force of the running motor when the electric vehicle runs downhill by gravity or when the vehicle is towed after a short-circuit occurred in the inverter device. Specifically, a control structure is disclosed in which, if the number of rotations of a running motor is equal to or higher than a reference value, it is determined that the electric vehicle is being towed or running downhill by gravity, and a main contactor, connected between a power source such as a battery and the inverter device, is kept off, so as to prevent any damage to the contactor.
Further, Japanese Patent Laying-Open No. 8-33103 (hereinafter referred to as Patent Document 3) discloses a control structure limiting discharge from a smoothing capacitor connected in parallel with the inverter device forming the motor driving means, when a towing state, in which the electric powered vehicle is towed, is detected. This prevents heating of a discharging circuit of the smoothing capacitor in the towing state, in which regenerative power of motor generates.
Japanese Patent Laying-Open No. 2006-87175 (hereinafter referred to as Patent Document 4) discloses a vehicle controller that issues an instruction to the driver to inhibit towing of the vehicle and operates a brake to stop tow-running, if a short-circuit occurred in a motor drive circuit or the like.
As described above, in an electric powered vehicle having a permanent magnet type motor as a motor for driving the vehicle, if a driving wheel rotates in tow running, for example, a rotor in the permanent magnet type motor coupled to the driving wheel rotates and generates back electromotive force. This results in short-circuit current in a short-circuit loop formed by the short-circuited switching element, an anti-parallel diode of a different phase, and a coil winding of a motor generator. Particularly, when the short-circuit failure of the switching element occurs only in one or more phases and not in all phases of the inverter device, the short-circuit current flows concentrated in a specific phase, resulting in excessive current and possibly results in more significant damage to the device.
The short-circuit current between the motor generators and in the inverter device as such cannot be prevented by the structures of Patent Documents 1 to 3. Patent Document 4 inhibits towing of the vehicle without exception when short-circuit failure occurs and thereby prevents damage to the device caused by excessive short-circuit current, while disadvantageously lowers convenience to the driver.